The antidepressant-sensitive norepinephrine (NE) transporters (NETs) constitute the major mode of synaptic inactivation of NE. Recent clinical genetic studies by our groups identified a coding mutation, A457P, in one NET allele of a proband with Orthostatic Intolerance (OI) presenting with reduced NE clearance, increased spillover and reduced intraneuronal NE metabolism. The A457P mutation was found to track with measures of postural tachycardia in the proband?s family and to correlate with altered synaptic NE metabolism. In Specific Aim 1, we propose to ascertain the functional impact of the A457P and other identified NET coding mutations in terms of transport and efflux, transporter trafficking and surface expression using heterologous expression systems. Evidence will be sought to support a dominant-negative interaction between mutant and wildtype subunits and whether homomultimeric complexes support NET function. In Specific Aim 2, we propose to extend our genetic evaluation of NET deficiency to evaluate additional subjects with OI and cardiomyopathy (CM) using high-throughput gene scanning techniques. These studies will focus on the NET coding exons and splice junctions and also include a recently identified intronic region that plays a critical role in NET gene expression. Methods will be implemented to allow for an evaluation of altered NET protein in biopsies tissue. Finally, attention and mood are dependent on proper noradrenergic signaling in the CNS and symptoms are present in our A457P probands indicating attention deficit, anxiety and hyperarousal. Thus, we propose in Specific Aim 3 to examine NET alleles with primary diagnoses of attention-deficit hyperactivity disorder (ADHD), attentional deficit (ADD) subtype and Major Depression, melancholic subtype, which is characterized by hyperarousal and anxiety. We will select subjects for analysis in both cases on the basis of comorbid tachycardia. Together these studies offer an opportunity for a better understanding of the molecular and behavioral manifestations of genetic NET variation.